Method of and means for recording sound



May 14, 1935. A. WHITAKER El AL 2,001,005

METHOD OF AND MEANS FOR RECORDING SOUND Filed June 4, 1931 Fig.4.

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Patented May 1 4, 1935 UNITED STATES PATENT OFFICE METHOD OF AND MEANS FOR RECORDING SOUND Application June 4, 1931, Serial No. 541,980 In Great Britain June 4, 1930 6 Claims.

The present invention relates to the recording of sound with the aid of light, and more particularly to those methods in which the sound is recorded upon a moving strip of light sensitive For simplicity, the sound record carrier will be termed a film, and the path on the carrier over which variations of amplitude may be recorded will be termed the sound track.

It-has hitherto been common practice in the recording of sound with the aid of light, to obtain a sound track of constant width, part of which is uniformly transparent and part uniiormly opaque, the dividing line between the two parts being a substantially true representation of the wave form of the sound. The whole of the opaque part of the sound track, which may be termed the sound line, is thus of varying width.

It has been found that if, during the process of reproducing from such a record, the film departs slightly from its motion in a straight line, that is to say, if the film wobbles or we-res" slightly, then the tips of waves on the sound line may arrive opposite the reproducing slit either before or after their proper timeor may be thrown beyond the end of the slit, with consequent distortion of the reproduced sound. Clearly, the larger the amplitude of the waves passing the reproducing slit, the greater is the distortion due to weaving of the film. Furthermore, if copies of such records are to be made upon opaque paper or other material by an ink printing process, as is sometimes desirable, irregularities may exist at the edge of the sound line due, for example, in some cases to a slight running of the ink, and these irregularities may sometimes be of the same order of magnitude as the radius of curvature of the tips of waves of comparatively high frequency and large amplitude. The tips of such waves may not therefore, be printed correctly and sounds reproduced from such ink printed records possess a further distortion in addition to that due to weaving.

It is an object of the present invention to provide a sound record of the varying width type, in which the above mentioned disadvantages are avoided.

According to the present invention a sound record is made in the form of a plurality of separate, relatively opaque sound lines of varying width.

It will be clear that since, on reproduction, the amplitudes of all the sound lines are added together the amplitudes recorded at all points on each sound line may be made less than that which would be necessary if the sound were recorded in the form of a single sound line, and so distortion due to weaving of the film may be reduced and the radii of curvature of the tips of the waves of the sound line never become too small for accurate printing.

The invention will be described with reference to the accompanying drawing, in which Fig. 1 shows, diagrammatically, one form of apparatus in accordance with our invention,

Fig. 2 shows, also diagrammatically, an alternative form of apparatus,

Fig. 3 is an elevation of a lens mounting used in connection with one form of our invention,

Fig. 4 shows a known type of sound record, and

Fig. 5 shows the type of sound record obtained with apparatus according to this invention.

Referring to Fig. 1, light from a suitable source I is focused by a lens 2 upon a curved oscillograph mirror 3, which is oscillated in any known or suitable manner in accordance with the sounds to be recorded, about an axis perpendicular to the plane of the paper. Between the lens 2 and the mirror 3 is placed a grid 4 comprising a number of apertures and stops, so that light falls upon the mirror 3 in the form of a number of similar beams of rectangular section. Light reflected from the mirror 3 falls upon a second grid 5 which is so situated in the path of the light beams, that, when the mirror 3 is in its mean position of oscillation (or position of rest), half the length of the apertures of the grid 5 are illuminated by the beams refiected from the mirror 3, and. the latter is so curved that an image of the first grid 4 is focused upon the second grid 5. A microscope objective 6 focuses a reduced image of the grid 5, together with the image of the grid 4 formed at 5, upon a film 'l which moves in a direction parallel to the pivotal axis of the mirror 3 and in a direction perpendicular to the direction of oscillation of the light beams. In order that loss of light may be reduced to a minimum, a lens 8, placed close to the grid 5, focuses an image of the mirror 3 onto the objective 6, Since it is desirable that the images formed upon the moving film 1 should be very narrow com pared with their length and that the maximu amount of light shouldreach'the film, the ap tures in the grid 5 are made narrower (in direction perpendicular to the plane of the paper) than those in the grid 4; and in order that as much as possible of the light reflected from the mirror 3 should be transmitted by these narrow apertures, a cylindrical lens 9, adapted to focus the beams down to the width of the apertures in the grid 5, is placed between the mirror 3 and the grid 5.

It will be'apparent that as the mirror 3 oscillates in accordance with the sounds to be recorded, more or less of the length of each aperture in the grid 5 will be illuminated by the beams of light refiected from the mirror 3. Since the film 7 moves in a direction perpendicular to the plane of the paper, there will be formed (after development of the film) a plurality of similar sound lines, one alongside of the other, upon the surface of the film, as shown, for example, in Fig. 5. If there are n oscillating light beams and n apertures in the second grid 5, there will be n sound lines recorded upon the film and in this case the amplitude of vibration of the reflecting mirror is arranged to be l/n that which would be required if the sound were recorded in the form of a single sound line.

It will be clear that, owing to the reduction in the amplitude recorded, the radii of curvature of the tips of waves never become very small and for this reason errors due to weaving of the film are reduced.

It will be apparent that the invention may be carried out during a re-recording process, in which the sounds are recorded, in known manner, in the form of a single sound line, electrical oscillations corresponding to the sound being then produced from this record and used to control the motion of the vibrating mirror.

In an alternative method of carrying the invention into effect, a record of the sound is obtained in the form of a single sound line of varying width, and several sound lines upon one track are obtained by a projection printing process carried out in the following way:

Referring to Fig. 2, a negative film l bearing a record of the sounds in the form of a single sound line (asillustrated in Fig. 4) is passed over a guide H which is so shaped that the filmv ill lies in a plane while passing the gate AB. Light from the source l2 illuminates the film portion which lies in the gate AB and passes through a plurality of slits l3, l4, IS in a grid IS, the latter being placed as close to the film I0 as is conveniently possible. The lengths of the slits in the grid l6 are perpendicular to the plane of the paper. Light transmitted through the slits in the grid 16 is focused by a plurality of lenses ll, [8, l9 upon a positive film 20 which moves, in the same direction and with the same speed as the film ID, over the guide 2| past the gate CD. As indicated in Fig. 2, the vertical distances between the centers of the lenses l1, i8, i9 is equal to the distances between the slits i3, i l, 15, but, while the horizontal distances (that is to say the distances in a plane perpendicular to the plane of the paper) between the centers of the slits l3, l4, l are zero, the horizontal distances between the centers of the lenses ii, i8, i9, are such that the images of the slits 13, M, 5 which are formed onthe positive 20, lie alongside one another. A detailed view of the relative positions of the lenses is shown in Fig. 3, the distances between their centers being greater in a direction parallel to the direction of motion of the film it} than it is in a direction perpendicular thereto. Referring again to Fig. 2, the focal lengths and positions of the lenses are so chosen that the images of the slits is, it, i5 formed on the positive 28 are equal and reduced in size. In order to minimize loss of light, a spherocylindrical lens 22 is placed close to the gate IS in such manner that all light transmitted by the slits l3, l6, l5 falls upon and fills the lenses IT, l8, Hi.

There are thus obtained three similar sound lines lying side by side as illustrated in Fig. 5, and, if the optical systems have been carefully arranged, they will be free from stagger. Since the negative I0 and positive 20 move with equal speeds, the frequency recorded upon the negative will be reproduced faithfully upon each sound line of the positive, but since each sound line is a reduced image of the negative, the amplitude of each sound line will be less than the amplitude recorded upon the negative. If there are m sound lines, each image of the negative produced at the positive must be reduced m times by the lenses ll, l8, l9.

If desired, it may be arranged that the tips of the sound lines always point towards the center of the sound track and in this way errors due to weaving of the film are still further reduced.

Although the invention has been described as applied to the recording upon, or reproducing from, transparent record carriers, it will be evident that it applies equally well to the case of opaque record carriers such as paper strip. In order that the foregoing specification may apply to opaque record carriers, the word transparent should be replaced by the words highlyreflecting and the word opaque should be replaced by the words slightly-reflecting". Clearly, also, the invention applies to the recording upon, or reproducing from, record carriers which are in the form of a drum or a disc.

We claim as our invention:

1. In apparatus for forming a photographic sound record, a light source, means for directing light from said source through a master sound record, means for dividing said light into 11" paths after passing through the record, optical means for projecting n transversely reduced images of the record, one in each of said paths, onto a photosensitive surface, the .said optical means being so constructed and arranged that the amplitude of each reduced image is of the order of l/n the amplitude of the original sound record whereby the sum total of the amplitudes of any given set of images, measured transversely of the record, is equal to the amplitude of the corresponding portion of the master record.

2. In apparatus as claimed in claim 1, an optical system comprising a plurality of lenses disposed in a plane parallel to the plane of said record, said lenses being displaced relative to one another so as to project reduced images of said record in substantial alignment on said light sensitive surface.

3. The method of forming a multiple trace photographic sound record from a single trace record which comprises passing light through said single trace record, subsequently dividing the light into n paths, and thereafter projecting n reduced images of said single trace record, one in each of said paths, in alignment with each other onto a photosensitive surface, while maintaining the amplitude of each reduced image substantially l/nthe amplitude of the single trace record whereby the sum total of the amplitudes of any given set of images, measured transversely of the multiple trace record, is equal to the amplitude of the corresponding portion of the single trace record.

4. The method of producing a quasi-variable density linear sound record from a master linear record whereon the recorded sounds are represented by a serrated line of demarcation between two areas having differing optical characteriatics, the amplitude and spacing of the sierra-- tions being functions, respectively, of the amplitude and frequency of the sounds, which comprises illuminating such a master record and projecting onto a photosensitive element a plurality of closely adjacent transversely reduced images thereof while maintaining a predetermined relation between the linear spacing of the peaks of the image serrations and the linear spacing of the peaks of the serrations on the master record.

5. The method set forth in claim 4 characterized in that the linear spacing of the image serration peaks is maintained the same as the linear spacing of the peaks'of the serrations on the master record.

6. The method set forth in claim 4 additionally characterized in that the total of the amplitudes of any given set of image serrations, measured transversely of the record, is maintalned of the same order of magnitude as the corresponding serration on the master record.

ALFRED WHITAKER. CECIL OSWALD BROWNE. 

